It has been known to concentrate liquid in evaporators by passing the material to be concentrated or subjected to evaporation through the heating and evaporation tubes while the tubes are heated from the exterior and the material to be evaporated releases steam or vapor which is recovered from the evaporation tubes. However, the evaporators have difficulties in achieving highly effective evaporation. To permit the aqueous liquids to be evaporated to a relatively high final concentration, theses evaporators apply complicated structural designs that allow prolongation of the stay time of the aqueous liquid in the evaporator. However, prolongation of the stay time increases energy consumption and is more likely to cause the decomposition of the thermosensitive material in the aqueous liquid.
In the industrial-scale production, catalysts used in the reactions are expensive, which results in relatively high production cost. To reduce the production cost, the catalysts are often recovered and reused. However, some catalysts are thermosensitive and instable; it becomes very difficult to separate and recover the catalysts without going through complicated operations.